This invention relates to the hydrogenation of heavy hydrocarbons, and especially to the hydrogenating pretreatment of heavy hydrocarbons before they are subjected to thermal cracking to produce normally gaseous olefins, wherein the hydrogenation product is separated into a lighter fraction and a heavier fraction; the heavier fraction is introduced at least in part to the thermal cracking step; and the lighter fraction is withdrawn as a product.
Light starting materials, i.e. hydrocarbons having a boiling point of below 200.degree. C., e.g. naphtha, are especially suitable for the thermal cracking of hydrocarbons to produce normally gaseous olefins such as ethylene and propylene. Such starting materials lead to high cracking yields and result in only a small quantity of undesired by-products.
However, in view of the high demand for olefins, which may lead to a short supply and increase in price of the aforementioned advantageous starting materials, several attempts have been made through the years to develop processes which permit the utilization of higher-boiling starting materials.
When employing such higher-boiling charges, the olefin yield is reduced and the yield of liquid hydrocarbons boiling above 200.degree. C. is increased. The proportion of the latter high-boiling fraction, which is difficult to treat in further operation, increases significantly with the boiling point of the starting material. In addition, further difficulties are encountered in that higher-boiling starting materials lead to increased formation of coke and tar. These products are deposited on the walls of the conduit elements, for example, pipelines and heat exchangers, thereby impairing heat transfer, and furthermore resulting in constrictions in cross section. It is therefore necessary to conduct a removal of these deposits more frequently than when using light hydrocarbons.
To solve this problem, it has been known to catalytically hydrogenate heavy hydrocarbons prior to thermal cracking. Thereby the content of those aromatic compounds leading to the undesirable cracked products is reduced in the starting material. Moreover, the starting material is also desulfurized. To further improve this conventional method, it has been suggested in assignee's allowed copending U.S. application Ser. No. 082,453, filed Oct. 9, 1979, incorporated by reference herein, that the hydrogenation product be separated into two fractions of differing boiling ranges and that only the heavy fraction be introduced into the thermal cracking step. The light fraction, boiling in the gasoline range, can be used, according to this process as gasoline owing to its relatively high degree of isomerization. However, the octane number of this light fraction is so low that catalytic reforming is required to raise the octane number for engine use.